An aircraft inertial navigation system is typically integrated with a global navigation satellite system (GNSS) receiver to provide reliable and continuous location information to assist the aircraft in performing various maneuvers, particularly when the aircraft is landing. In such systems, the information from the inertial navigation system and the information from the GNSS receiver are integrated together using a Kalman filter. The Kalman filter generates high integrity and continuous location information. The use of Kalman filters, and other types of filters, are well known.
For example, U.S. Pat. No. 5,184,304 to Huddle, entitled “Fault-Tolerant Inertial Navigation System,” U.S. Pat. No. 5,583,774 to Diesel, entitled “Assured-Integrity Monitored-Extrapolation Navigation Apparatus,” and U.S. Pat. No. 6,549,829 to Anderson et. al, entitled “Skipping Filter For Inertially Augmented Landing System,” all of which are incorporated by reference herein in their entirety, are examples of such integrated navigation systems that use filters to provide high integrity and continuous location information.
The reliability of the location information provided by the GNSS receiver is a critical issue. If the location information provided by the GNSS receiver is inaccurate, or if the information is not available from the GNSS receiver, then sufficiently accurate location information may not be available to assist the aircraft in its performing various maneuvers. An exemplary fault mode may occur if one or more of the electronic components of the GNSS satellite, or the GNSS receiver fails, such that the location information becomes inaccurate. In such situations, location information from the GNSS receiver should not be used. Another exemplary fault mode may occur if the clock times used by the GNSS satellites drift and become sufficiently inaccurate so as to render the output location information inaccurate. Or, one or more of the GNSS satellites may not be in its expected orbital location, such as when a GNSS satellite moves in an unexpected or unknown manner from its designated orbital route, such that the determined location information based upon a signal received from the GNSS satellite is incorrect. Other fault modes may occur if the information from the GNSS system is not available. An exemplary fault mode in this case is one or more GNSS satellites signals are blocked from being received by the GNSS receiver, such that not enough GNSS signals are available for determining the user location information.